1. Field of the Invention
The present invention relates to a zoom lens barrel. More precisely, the invention relates to a zoom lens barrel having at least three groups of movable lenses.
2. Description of Related Art
There are basically two types of zoom lenses having three lens groups. A first type has a cam ring which is provided with cam grooves to move the three lens groups. A second type includes cam grooves formed on two or more cam rings, which are rotated in association with each other, for moving the three lens groups. However, in the first type, there is little freedom of design, since the three cam grooves must be formed on a single cam ring without interfering with each other. Consequently, it is necessary to increase the inclination angles of the cam grooves in order to reduce an angular displacemant of the cam ring from the shortest focal length to the longest focal length. However, the increased inclination angle of the cam grooves requires an increased force to rotate the cam ring. On the other hand, in the second type mentioned above, the provision of a plurality of cam rings requires a large space and the manufacturing cost is high.